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Old 4th May 2012, 12:09 PM   #1
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Question 6DJ8/ECC88-SRPP Tube Preamplifier

I want to make for a friend one 6DJ8/ECC88-SRPP Tube Preamplifier but I don't have on hand the 150uF, I have 100uF or 220uF, wich one can I use? have I to change the cathode resistor value?
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Old 4th May 2012, 12:10 PM   #2
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Ooops forgot the schematic.
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Old 4th May 2012, 12:11 PM   #3
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If there is a way to upgrade the schematic your advice will welcome.

Wich value of pot for volume, I have 100K it's ok?

Last edited by merlin el mago; 4th May 2012 at 12:16 PM.
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Old 4th May 2012, 12:16 PM   #4
SY is offline SY  United States
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Either don't use an SRPP or optimize the loading- in a very nice article a few years ago, Merlin Blencowe showed that unlike common-cathode voltage amps, an SRPP will show a distortion null at some (relatively low) value of load. As well, the gain is extremely high for a line amp- he'll probably clip his power amp with the volume control barely advanced.

The schematic has a couple of glaring errors (the input impedance and the bias voltage). You could use a larger cap at the expense of overload recovery time. Or you could substitute a cheap red LED to get near-instantaneous overload recovery, some pretty light, and slightly reduced cost.
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Old 4th May 2012, 12:34 PM   #5
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He has a clone Aleph X of 100W, what's the correct bias voltage?

I have on hand these LEDs:

1.8V 20mA Yellow water clear
1.9V 20mA Red
1.95V 20mA Yellow
2.0V 20mA Green
2.1V 25mA Greed diffused

Wich one do you recommend?

How can reduce the gain?
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Old 4th May 2012, 12:44 PM   #6
jane is offline jane  Norway
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The 0.1 uF output cap seems a bit small for full range audio -- but it depends on the load. For a 10k load fc(-3dB) would be 160Hz with the 0.1 uF cap.
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Old 4th May 2012, 12:46 PM   #7
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Quote:
Originally Posted by jane View Post
The 0.1 uF output cap seems a bit small for full range audio -- but it depends on the load. For a 10k load fc(-3dB) would be 160Hz with the 0.1 uF cap.
Thank you, do you know the input impedance of the Aleph X 100W?
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Old 4th May 2012, 12:47 PM   #8
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Quote:
Originally Posted by merlin el mago View Post
Wich one do you recommend?
A cheap red LED will give you about 1.7V at the (roughly) 10mA this circuit runs, with very low noise and impedance.
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Old 4th May 2012, 01:00 PM   #9
DF96 is offline DF96  England
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The quoted LF point of 1.6Hz assumes no load (not even a scope probe!). I'm not sure where the input impedance of 240K comes from. The output impedance of 850ohms may be correct (I haven't bothered to do the calculation) but it will only apply above about 2kHz with a small output cap. Everything points to this design being done by someone who is new to audio and so ignorant of details.
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Old 4th May 2012, 01:24 PM   #10
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You can get rid of the 150uf cathode cap, that would save a part and decrease gain.
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